The epithermal class of deposits (including Chelopech) were originally classed as “massive sulphide copper pyrite deposits”. Recent studies indicate that an epigenetic origin for the mineralisation formed by the replacement of volcanic rocks is more suitable (Chambefort, 2005).

Current models (e.g. Hedenquist et al., 1994) for high sulfidation epithermal systems suggest silification was early and related to initial gas expansion (SO2, HCl) that separated from a denser, metal-bearing brine at depth. This gaseous solution mixed with meteoric waters to produce sulphuric acid. At Chelopech, multiple events related to both silification and mineralisation, were probably driven by pressure fluctuations, degassing and fault-valve activity above a metal-bearing brine fluid at depth.

Lack of stockwork chalcopyrite-bornite bearing quartz veins with sequential potassic–phyllic to propylitic alternation haloes, felsic intrusives and abundance of copper-arsenides at Chelopech suggests porphyry copper mineralisation is not present.

High arsenic-sulphur systems represent a change in fluid conditions which have commonly been observed in the youngest paragenetic stages of porphyry copper mineralisation. The fluids responsible at Chelopech are of a different character and are more acidic and possibly more reduced remnants of a de-gassed brine material, capable of chloride-gold transport. Therefore, porphyry copper conditions may have occurred further out in the Chelopech district, and (if not exposed) may be preserved at depth (RSG Global, 2007).

Three successive mineralisation stages have been recognised at Chelopech, including an early iron- sulphur stage consisting mainly of disseminated and massive pyrite, a second copper-arsenic-sulphur stage which is the economic copper and gold stage, and a late lead-zinc stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.

The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. The mineralised zones vary from 40 m to 200 m in length, are 20–130 m thick, and can extend at least 390 m down plunge. Sub-vertical vein mineralisation is volumetrically the most important mineralisation style at Chelopech (Chambefort, 2005).

Sulphide mineralogy is dominated by pyrite, marcasite, melnikovite, tennantite, enargite-luzonite, and chalcopyrite, together with subordinate famatinite, sphalerite and galena. In gross terms, about 45% of the copper is in the form of arsenides and sulfosalts, 50% as chalcopyrite and 5% as oxides.

Quartz, barite and kaolinite are the dominant gangue minerals with chlorite, ankerite and gypsum subordinate.

Gold occurs in a variety of forms, both as native metal with admixed silver in a stoichiometric form approximating to Au3Ag and in auriferous tellurides. The gold is fine grained (5–300 microns, with 5–20 microns the norm). Metallurgical studies have shown a significant proportion of the gold is refractory, typically:

• 45% intergrown within pyrite, chalcopyrite and sphalerite
• 25% intergrown with enargite, luzonite, tennantite, tetrahedrite and bornite
• 20% finely intergrown with chalcedonic silica
• 10% as free gold.

Silver-bearing rock and native silver are usually spatially associated or finely intergrown with pyrite and galena (62%) with enargite, tennantite and tetrahedrite (15%) and as electrum (23%).

Other major sulphides and arsenides exhibit simple crystalline and intergrown forms with the pyrite and occur in intra-crystal spaces as replacements, as replacements of pyrite, as crosscutting veinlets and as overgrowths. Intergrowths of the cupriferous minerals are commonplace, both as aggregates and as complex textures with several intergrown minerals.

Production from underground is attained via Sublevel Longhole Open Stoping. The various ore bodies are developed at nominal 30 m vertical intervals and accessed by major declines in both, the Western and Central areas. Stopes are designed to be 20 m wide between the levels. The length of the stope depends on the geotechnical conditions, but can range between 20 and 60 m. The new trend of stope design is to keep a 20-30m length and 60m height, where geological and geotechnical conditions are suitable. This allows for improvement in ore handling and dust suppression during ore mucking as a result of shorter remote loading. Ore is delivered via ore passes, or via trucks, to the run of mine (ROM) bin above the crusher. The crusher feeds up to 400 tph to a system of eight conveyors, to transport the ore to the surface stockpile.

Once mined via an “end-slice” methodology, stopes are backfilled with “paste fill” produced from the mill tailings, to which cement is added and which is gravity fed underground via a system of borehole and pipes to the stopes being filled.

Multiple horizons are designed in each ore body so that multiple stopes can be in production at any one time. Simulations have shown that at least six stopes shall need to be producing ore to maintain ore production of 2.2 Mtpa, with up to 22 stopes being drilled, “mucked” and filled at any one time.

Underground Crusher Conveyor System
The previous system of train haulage and shaft hoisting has been entirely replaced by a 2 Mt/a underground crushing and conveying system, that takes ore from an ore pass system underground reporting to the 195 level and crushes, transports by conveyors and discharges the ore onto a 6,000 t live capacity reclaim stockpile on surface.

The total length of the six underground conveyors is 3.9 km. Total lateral development required was 4.5 km. The design capacity of the system is 400 t/hr as a result of this underground crusher installation, the existing surface crusher installation is reduced to one crusher, to handle oversize and to maintain minimum production in case of emergency.

The new conveyor system was commissioned in 2012 and has been in operation continuously since then. This system is the only means of ore hoisting to surface.

The previous system of train haulage and shaft hoisting has been entirely replaced by a 2 Mt/a underground crushing and conveying system, that takes ore from an ore pass system underground reporting to the 195 level and crushes, transports by conveyors and discharges the ore onto a 6,000 t live capacity reclaim stockpile on surface.

This ore handling system incorporates a primary crusher (a 1,070 mm x 1,500 mm jaw crusher) between the 195 level and the 165 level underground, which discharges into a 400 t crushed ore bin. The crusher is fed from a ROM bin sitting under a grizzly with openings of 800 mm x 800 mm.

Ore is fed to the grizzly via three sources:
1. A 4 m diameter x 135 m long ore pass for 151 and 150 block material above the 260 level.
2. A 7 m diameter x 30 m long ore bin for the 144, 145, 147, 149, and 103 blocks, 150 and 151 blocks between the 225 and 260 levels; and the Central area 16, 18 and 19 blocks.
3. A truck tip directly on the grizzly for ore in 151 and 150 blocks, on and below the 195 level.

A plate feeder draws material from the 400 t crushed ore bin and loads a picking belt (CV1) for removal of tramp metal using a self-cleaning magnet. Material is then conveyed via six more haulage conveyors (CV2- CV7) to the surface. The surface conveyor (C1105) transfers this material to the surface reclaim stockpile, where it is reclaimed and conveyed to the SAG mill to provide uninterrupted feed to the process plant.

The total length of the six underground conveyors is 3.9 km. Total lateral development required was 4.5 km. The design capacity of the system is 400 t/hr as a result of this underground crusher installation, the existing surface crusher installation is reduced to one crusher, to handle oversize and to maintain minimum production in case of emergency.

Crushed product from the primary crushers, which has a typical P80 of 100 mm, is ground using a single-stage closed grinding circuit with cyclone classification. This comprises a single stage SAG mill, 8.53 m diameter x 4.72 m long, with a rated capacity of 5,800 kW. Cyclone underflow is returned to the SAG mill and the overflow gravitates to the flotation circuit passing via an “in-stream” analysing system, which monitors the density and the assay composition of the stream, and a particle size analyser.

Current ore treatment processes comprise conventional crushing of run-of-mine (ROM) ore in a primary jaw crushing circuit, grinding in a SAG milling circuit, bulk floatation, three-stage cleaner flotation and concentrate dewatering to produce the copper/gold concentrate, while the pyrite is recovered from the copper circuit cleaner tails.

The primary saleable product is a gold-copper concentrate containing, on average, 16.5% Cu, 35 g/t Au and 5.5% arsenic, which is loaded at the mine site through a conveyor system from the stockpile into rail wagons and dispatched to the Port of Burgas for sea transportation to the Company’s smelter in Namibia and to third parties.

Since 2014, pyrite concentrate, containing gold, has been produced in a section with a capacity allowing the production of up to 400,000 tonnes of pyrite concentrate per year from the mill feed as a separate secondary concentrate product, in addition to the produced gold-copper concentrate. Production i ........